SPS-03

STATE OF

ARKANSAS GEOLOGICAL SURVEY

Bekki White, State Geologist and Director

STATE PARK SERIES 03

GEOLOGY OF THE CRATER OF STATE PARK AND VICINITY, PIKE COUNTY, ARKANSAS

by

J. M. Howard and W. D. Hanson

Little Rock, Arkansas

2008

STATE OF ARKANSAS

ARKANSAS GEOLOGICAL SURVEY

Bekki White, State Geologist and Director

STATE PARK SERIES 03

GEOLOGY OF THE CRATER OF DIAMONDS STATE PARK AND VICINITY, PIKE COUNTY, ARKANSAS

by

J. M. Howard and W. D. Hanson

Little Rock, Arkansas

2008

STATE OF ARKANSAS

Mike Beebe, Governor

ARKANSAS GEOLOGICAL SURVEY

Bekki White, State Geologist and Director

COMMISSIONERS

Dr. Richard Cohoon, Chairman………………………………………....Russellville

William Willis, Vice Chairman…………………………………...…….Hot Springs

David J. Baumgardner………………………………………….………..Little Rock

Brad DeVazier…………………………………………………………..Forrest City

Keith DuPriest………………………………………………………….….Magnolia

Becky Keogh……………………………………………………...……..Little Rock

David Lumbert…………………………………………………...………Little Rock

Little Rock, Arkansas

2008

i

TABLE OF CONTENTS

Introduction…………………………………………………………………………...... 1 Geology…………………………………………………………………………………………... 1 Prairie Creek Diatreme Rock Types……………………………….…………...……...………… 3 Mineralogy of Diamonds…………………….……………………………………………..……. 6 Typical shapes of Arkansas diamonds…………………………………………………………… 6 Answers to Frequently Asked Questions……………..……………………………….....……… 7 Definition of Rock Types……………………………………………………………………… 7 Formation Processes.…...…………………………………………………………….....…….. 8 Search Efforts……………...……………………………...……………………...………..…. 11 Economic Concerns…………………………………………………………………………... 11 Identifications……………………….………………………………………………. 12 Misconceptions………………………………………………………………………………. 12 References and Reading List ………………….…………………..………………………...…. 13 Alphabetical Listing of reported from the Prairie Creek Diatreme and their Geologic Associations………………………………………………………………………….…...... 14

Figures

Figure 1. General location of the Crater of Diamonds State Park within Arkansas…..... 1 Figure 2 General stratigraphy of the local rock units……….………………..…...... … 2 Figure 3 Map and cross-section of Prairie Creek.…...... …………………….…. 3 Figure 4. Magmatic boulders on crest of Middle Hill.…..……………….… 4 Figure 5. Lamproite lapilli tuff exposed on West Hill………………………………….. 5 Figure 6. Altered lamproite breccia tuff…….…………………...………………...... … 5 Figure 7. Outcrop of silica-cemented sandstone-dominated epiclastics…..……… 6 Figure 8. dissolution evolution…………………….…………….………....… 7

ii GEOLOGY OF THE CRATER OF DIAMONDS STATE PARK AND VICINITY, PIKE COUNTY, ARKANSAS

by

J. MICHAEL HOWARD AND W. D. HANSON

INTRODUCTION Survey entitled “Geology of the Crater of Diamonds State Park and Vicinity, Pike The Crater of Diamonds State Park is County, Arkansas.” The reader can learn a remarkable in several ways. First, it is the lot of geology and geologic history only place in the world where anyone may pertaining to the sedimentary and igneous pay a small entrance fee, look for diamonds, rocks of the area, and specifically of the and keep what you find! It fascinates Park, from the poster and even more by geologists because the site presents a reading the contained information. The window into the geologic past and the geologic story is a fascinating one, not just earth’s mantle, a rare thing indeed. The park for geologists, but for anyone wishing to and surrounding area has a rich verbal and learn why the diamonds are present, their written history…history of the settlement of age, and the past environment that existed the region, of the discovery of diamonds, of when the diamondiferous igneous rocks the many attempts at commercial and tourist were explosively emplaced. Key questions development based on the presence of that to be answered also include what is so , and history of preservation and unusual about these rocks, why do they continued development since it became a weather so rapidly, how old are the rocks state park and, as such, public property. and the diamonds, along with many other questions. The authors hope you enjoy your travels through geologic time as we will be your guides.

After the geology and mineralogy sections and before the reading list are frequently asked questions. It is hoped the answers to these questions will assist the visitor in better understanding the geology of the Park.

GEOLOGY

Crater of Diamonds State Park is just south of the in Pike County,

Arkansas and along the northern margin of Figure 1: General location of the Crater of the West Gulf Coastal Plain. Diamonds State Park within Arkansas. The Ouachita Mountains region is an area of This publication is written to accompany the major disturbance on the southern margin of geologic poster by the Arkansas Geological what is now called the North American

1 continent. Sediments that were deposited in sedimentary rocks were a deep ocean basin south of the continental deposited on the southern margin of the margin, dating from 500 to 245 million Ouachita Mountains and range in age from years ago, were shoved up and onto the 144 – 66.4 million years. They represent continent and were “welded” to the sediments that were deposited in very continental mass by the end of the shallow on the northern margin of the mountain-building episode at the close of Cretaceous seas. The beds dip gently to the time. Since then, south and now outcrop from near and erosion have taken their toll on this east- Arkadelphia in Clark County west to the west mountain chain, resulting in a subdued Arkansas-Oklahoma state border near relief relative to when it first was formed. DeQueen in Sevier County. Cretaceous rocks are displayed on the poster and chart in varying shades of green. The Cretaceous units have been eroded by local rivers and streams.

Deposits of those rivers and streams during the date from 1.6 million years ago to recent times and are displayed in shades of yellow on the poster. Both Cretaceous and Quaternary units are located within the West Gulf Coastal Plain region of Arkansas.

Along this coastal margin, about 100 million years ago, several explosive events happened, resulting in the emplacement of diamond-bearing rocks that geologists term lamproite breccia and tuff as a diatreme. A diatreme is a breccia-filled

formed by a gaseous explosion. The source Figure 2: General stratigraphy of the local of rocks that compose the exposures of the rock units. Prairie Creek pipe, exposed on the surface at the Crater of Diamonds State Park, and the The oldest rock unit displayed on the poster other known bodies of similar igneous and on the stratigraphy chart (fig. 2) outcrops materials to the immediate northeast was the north of Murfreesboro and is named the earth’s mantle. Some field evidence Jackfork Sandstone. Its mapped extent is indicates that this pipe was emplaced on the represented by the color gray. Locally, the seaward side of the ocean-land margin, sandstone beds dip steeply to the south due perhaps only a very short distance off shore. to their distortion during the Ouachita Soon after the explosive event, magmatic (mountain building episode). lamproite, arising through the explosive Where unweathered, it has been a major vent, reached the surface and created lava- source of crushed sandstone for aggregate. filled lakes within the existing crater.

2 Afterward, geologic processes became less Prairie Creek Diatreme Rock Types catastrophic and the ongoing active cycle of deposition along the continental margin Surface exposed rock types of the diatreme, during the Cretaceous buried the volcanic may be divided into three general groups: vents. Only late in the Quaternary did the magmatic lamproite, pyroclastic lamproite, Little Missouri River erode those sediments and maar epiclastics. Magmatic lamproite capping the pipe and expose the larger pipe contains only very few micro-diamonds, to erosion (fig. 3). From field relationships whereas pyroclastic lamproite is the source with the adjacent pipes, geologists estimate of the soil from which all the diamonds are that only about 165 feet of erosion has taken recovered. The maar epiclastics are thought place on the Prairie Creek pipe since its to contain some diamonds but have not been emplacement. It is geologically remarkable tested to determine their content. Data to have some of the rock types at the Crater gathered during the 1990’s exploration work of Diamonds State Park exposed. indicates a deep weathering and alteration Worldwide, few of these types of pipes have zone of some 40 feet or more. Unaltered the upper layers of rock material preserved. rock samples are therefore unavailable by Most have suffered intensive weathering and surface sampling. deep erosion.

Figure 3: Map and cross-section of Prairie Creek. Modified from Morgan, 1993.

3 Magmatic Lamproite examination of thin sections of these rocks allows further descriptive terminology such Originally termed by early as lamproite lapilli ash tuff and lamproite investigators and later hypabyssal ash lapilli tuff. The soils derived from these lamproite and () units are the source of the diamonds. These madupitic lamproite by recent workers, the units are not divided on the bedrock geology rock is a magmatic non-explosive phase map. These rocks weather rapidly because material. The rock is somewhat resistant to most of the minerals composing them weathering and consists of crystals and formed at great depth and are very unstable crystal fragments of olivine, in various at surface temperatures and pressures. stages of alteration to serpentine, set in a fine matrix of , Lamproite Lapilli Tuff diopside, , and . Magmatic lamproite may contain This rock represents fine-grained ash and of rocks from the mantle and crust, brought dust that was formed by the initial explosion along by its movement to the earth’s surface. and settled as air fall material. Some of this Good exposures of this rock may be seen material exhibits cross-bedding, indicative immediately north and down slope from the of the presence of water on the site that has old mine shack within the search area and either reworked the original ash or that the along Prospectors Trail where it forms ash was deposited in water and then bouldery masses on the crest and north side reworked. Tuff varies greatly in texture of Middle Hill (fig. 4). from one hand specimen to another. Some specimens consist of grains and fragments that average less than 1 mm (.04 inch). Such rocks appear even-grained. Other specimens may contain fragments and grains up to 1 cm (0.4 inch) in diameter with much finer grained matrix (fig. 5). Finally, a few samples contain a predominance of these larger grains and little fine-grained matrix. Phlogopite mica is present in all samples, often a major component, and typically is fine-grained. Also present are many small white to pale yellow grains of serpentine,

probably representing weathered olivine. Figure 4 : Magmatic lamproite boulders on Exposures of this rock are present in the crest of Middle Hill. southwestern portion of the search area, as displayed on the poster. A significant Pyroclastic Lamproite number of diamonds have been recovered from soils developed on this rock type. This rock was originally termed by early workers simply because it contained diamonds. Pyroclastic lamproite may be subdivided generally into two field identifiable types: lamproite lapilli tuff and lamproite breccia tuff. Microscopic

4 probably from air fall sorting or grain fluid flow.

Figure 5 : Lamproite lapilli tuff exposed on West Hill. Altered xenoliths of shale and other rock types are scattered throughout Figure 6 : Altered lamproite breccia tuff. this exposure. Tan to whitish spots are serpentine after olivine. Lamproite Breccia Tuff Maar Epiclastics This rock is composed of fragments of lamproite and other rocks that were both Maar epiclastics were originally described shattered during rapid transport to the as -bearing tuffs. They formed by surface and the initial near surface explosive admixture of uncemented Cretaceous sands emplacement event. In hand specimen, it is and clays with the lamproite lapilli and difficult to find a truly hard specimen, since breccia tuff units. -cemented exposures weathering has altered the fine-grained that are predominated by silica-cemented matrix and most of the breccia fragments. sand grains are exposed in several locations Olivine was a major component, now mostly on the pipe. Exposures of this erosion altered to serpentine. Some breccia resistant rock are present on the east edge of fragments of lamproite still contain glassy the search area, adjacent to and south of the olivine crystals, only having marginal Kimberlite Café (fig. 7), and as bouldery alteration. The majority of diamonds residuum on the crest on the northeast end of recovered thus far have been from soils and East Hill. The Diamond Discovery Center residual materials left from the weathering also sits atop this rock unit. Diamonds are of the lamproite breccia tuff. In hand expected to be present within soils derived specimen, the rock is dark brownish with from epiclastic rocks, but their abundance is scattered yellow, tan, or whitish spots of unknown. Diamonds will certainly be a serpentine after olivine set in a fine grained lesser constituent of this rock, simply due to matrix (fig. 6). Specimens may contain the admixture of non-diamondiferous varying sized sub-rounded rock fragments of materials with the original diamond-bearing lamproite that contain small glassy olivine tuff units. crystals. It is difficult to find a fresh rock of this type that cannot be readily broken apart by hand, due to rapid alteration and weathering. A few hand specimens of this rock type exhibit crude stratification,

5 ago) and on the basis of dating of phlogopite mica (106 + 3 million years ago).

Diamonds reached the surface through the rapid rise of the diatreme, material from the diamond-rich zone being captured and swept along during the ascent process. Laboratory experiments indicate that diamond is not a stable mineral during the rise to the surface and that this rise must be very rapid to prevent them from being completely dissolved into the matrix rock. Speeds on the order of 60 to 250 miles per hour are speculated by researchers as being required. Diamonds are stable at conditions within the within certain limitations of temperature and pressure. The amount of time necessary to reach the surface from this zone of stability and still retain diamond in the matrix is thought to vary from one to only several hours. Certainly from the time they are captured and swept along until they reach the surface is less than 1 day.

Figure 7: Outcrop of silica-cemented Typical shapes of Arkansas diamonds.

sandstone-dominated maar epiclastics. Images courtesy of Glenn Worthington. Northeastern border zone of Prairie Creek

diatreme.

MINERALOGY OF DIAMONDS

Diamonds at the Crater of Diamonds State Yellow, 50 point Pale yellow, 33 pt Brown, 7 pt Park originated in the mantle as part of the early formation and crystallization of the earth, after the separation of the earth into the core, mantle, and crust. As crystals, these diamonds would have been in the form of octahedrons, and more rarely dodecahedrons and cubes. From mineral White, 26 pt Pale brown, 16 pt inclusions present within the diamonds that were captured during crystal growth, isotopic age dating places the diamonds at approximately 3 billion years old. The formation of the pipe itself has been dated from local stratigraphy as upper Early

Cretaceous (113 + 4 to 97 + 2.5 million years Brown, 16 pt Pale yellow, 6 pt White, 4 pt

6 What is Lamproite? Lamproite is a rock name that describes highly potassic and somewhat aluminum- poor igneous rocks with the following range of minerals: phlogopite, a variety of tremolite called , leucite, , diopside, and a number of rare -, barium-, -, and zirconium-rich oxides and silicates. Leucite and sanidine are not present at the Park. This is a rare type of rock when considering the abundance of all the igneous rocks exposed Figure 8 : Diamond dissolution evolution. on the earth’s surface. (Khokhryakov, A.F., and Pal’yanov, Y.N., 2007) form from partially melted Conditions are not ideal for diamonds during mantle at depths exceeding 93 miles. The the ascent. At the Park most diamonds have rising material is forced to the surface in suffered some dissolution by the matrix and volcanic pipes, bringing with it xenoliths transporting fluids, and perhaps even and xenocryst diamonds from the breakage and dissolution effects. Rice grain harzburgitic peridotite or mantle and flattened shapes are not uncommon and regions where diamond formation is probably represent shards broken from stabilized. In the instance of the Arkansas octahedral diamonds and/or resorbed macle lamproites, the diamonds are of eclogite host twins. The shapes of diamonds recovered origin. Eclogite is a rock consisting of red from the Park (above) appear on the , green clinopyroxene, and diamond dissolution evolution chart in the trace minerals, diamond being one. area of 60 to 80 percent weight loss (fig. 8). A significant amount of the deposit’s Lamproite intrusions form either champagne- diamond contained within the host rock has or martini-glass shaped diatreme masses, as been lost during transport due to dissolution well as dikes. processes. That is significant factor in a commercial venture because there must be a What is the difference between lamproite high enough remaining diamond value to be and kimberlite? recovered by modern extraction methods to Kimberlite is a name applied to a group of be profitable. The value of mined diamond volatile-rich, mainly , rough is set by world markets and depends potassic ultrabasic igneous rocks with a on average size, overall quality, and range of visible mineral crystals set in a percentage of gem diamonds to industrial fine-grained matrix or groundmass. Most of diamonds. these visible crystals did not form in the groundmass but were captured by it during ANSWERS TO FREQUENTLY ASKED transport. These crystals may consist of , pyrope garnet, olivine, QUESTIONS clinopyroxene, phlogopite, , and

. The matrix mineralogy is complex, Definition of Rock Types Definitions of but may include olivine, phlogopite, Lamproite/Kimberlite: Klein and Hurlbut perovskite, , and diopside. (1993), p. 348.

7 Kimberlite pipes are the result of explosive Formation Processes diatreme volcanism from very deep mantle derived sources. The theory is that they are How was the crater formed? formed deep within the mantle, between 93 Around 100 million years ago, a mass of and 280 miles in depth, from anomalously material moved out of the earth’s mantle at a enriched exotic mantle compositions, and high rate of speed…30 to 50 miles per hour, are erupted rapidly and violently, often with driven by carbon dioxide gas. During its considerable CO2 and volatile components. journey, it gathered rocks from all the zones It is this depth of melting and generation of rock it passed through. As it reached the which makes prone to hosting upper crust, the speed increased to as much diamond xenocrysts. as 150 miles per hour. When it came near the earth’s surface, about 850 feet below the Contrast this information to the description ground, the pressure of the gas overcame the of Lamproite, given above. Both kimberlite weight of the overlying sedimentary rock and lamproite have similar gas driven and an explosion occurred. The expanding mechanisms during transport to the surface, gas rapidly cooled the mass so we see little however, kimberlites often contain minerals heating effects in the surrounding sediments. from greater depths than do lamproites. The resulting explosive is classified Kimberlite intrusions form carrot-shaped as a diatreme by geologists. diatreme masses and dikes. Where is the volcano now? When did the names change from When you walk out onto the Collecting Area peridotite and kimberlite to lamproite and you are actually standing on it! The Prairie why? Who was responsible for the Creek diatreme did not form a classic cinder change? cone shape because it is not that type of After the discovery of the Argyle pipes in volcano, but instead blew out an inverted Australia, researchers realized that diamonds cone-shaped mass of material, much of were present in a rock that was not which fell back into the site. Erosion has kimberlite. So they came back and sampled removed about 165 feet of the upper portion all the known rocks called of the intrusion since the end of volcanic kimberlite/peridotite. In 1977, two ladies, activity. Evidence from commercial B. H. Scott Smith and E. M. W. Skinner, exploration indicates the sides dip toward published a paper entitled A New Look at the center of the 80-acre exposure at about Prairie Creek, Arkansas in a special book on 45 degrees, resembling a martini-glass. kimberlites and related rocks. In that paper, they proposed the rock name changes based How deep is the volcano? on their detailed work on the minerals The roots of the mass extend into the present. The new name more accurately mantle, to at least 93 miles deep through a describes to geologists the type of rock very narrow feeder tube, but the largest mass present at the Prairie Creek diatreme. of the volcano is within 660 feet of the Scientists in general and geologists in earth’s surface. particular are responsible for the name change because they have now accepted the How does a diamond form? new name and use it instead of the old ones. In order for diamonds to form, they require Science is open to change depending on the extremely high pressures and temperatures discovery of new evidence. which are only found deep in the earth’s

8 mantle. Diamonds are the stable form of contain diamonds and are similarly derived carbon at temperatures and pressures that from the upper mantle. However, both exist there. Under those conditions they peridotite and eclogite rock fragments crystallize into the cubic form of commonly come apart during the carbon…diamond. Isotopic age dating of emplacement process resulting in a matrix mineral inclusions captured by diamond as it containing the disaggregated minerals of formed implies that diamonds are much olivine, pyroxene, garnet, and diamond older than the age of the intrusion; in fact, (xenocrysts). During the movement of the some 3 billion years old. This means that diatreme from the mantle to the earth’s they formed long ago in the earth’s history surface, the diatreme passed through and were brought to the surface much later. diamond-bearing zones in the mantle. These diamond-bearing rocks, eclogite and Despite the popular belief, diamonds are not peridotite, are captured and swept along forever! They are metastable crystals at the with the mass, many of which come apart earth’s surface conditions, but have carbon while being transported. Consequently, atoms so tightly packed together that many of the minerals present in the without an input of energy, they cannot lamproite tuffs did not form by normal readily convert to graphite, a more stable magmatic processes of crystallization during form at surface conditions. transportation, but instead are foreign grains. Diamond, garnet, and olivine are examples. It is interesting to note that diamonds from the Crater are all well rounded, a testament Why are the diamonds so small, averaging to the chemically corrosive conditions they ¼ carat? encountered during their trip from the upper Most carbon in the mantle is thought to be in mantle to the surface. What typically are the form of carbonate minerals and called crystal faces are actually absorption diamond, particularly in the upper mantle features, readily recognized by their curved which has been enriched in carbon by faces and curved lines at face joins. The mantle plume or heat transfer processes. diamonds have lost approximately 60 to 80 Carbon that was available in the rock to percent of their weight during transport, due crystallize into diamond was not abundant, to this process. however, because we are dealing with crystallization processes that took place long How did the diamonds get here? before upper mantle enrichment. During Although diamond crystals are found in crystallization, carbon is removed from the lamproite and related rocks, the origin of liquid and attached to the orderly structure diamond is more closely related to the of the forming diamond. The liquid fragments of peridotite and eclogite which surrounding a given crystal is therefore are derived from the upper mantle, below depleted in carbon. If this happens under cratonic (shield) areas. It is here that the relatively uniform conditions, like a static rock, eclogite, forms. Eclogite consists of state, then many small diamonds form rather red pyrope garnet and green clinopyroxene, than a few large diamonds. Also, working along with diamond crystals that developed against the diamonds being discovered as with the garnet and pyroxene crystals. large crystals is the process of their transportation at a later time to the earth’s Peridotite fragments (xenoliths) composed surface. The material they are mixed with of garnet, olivine, and orthopyroxene also during their movement is corrosive to the

9 entrained diamond crystals, so if the crystalline structure to break the light into its transport speed is not very fast, the colored parts, much as a prism. So we see diamonds begin to be resorbed into that flashes of red, blue, yellow, green, and material and get smaller and smaller. If orange from the reflected light. transport is too slow, then they disappear, like salt crystals when dissolved in water. Why are these minerals present on the At the Crater, diamonds are well rounded Crater? due to resorption and have lost a lot of their The suite of minerals present at the Park size. Because diamonds are rare and take represents the culmination of many different some effort to find, discovering even a small and varied geologic processes that have one is an exciting experience! acted on the area. Sedimentation and cementation of the sedimentary host rocks, Why do diamonds have an oily film on silicification and deposition of silica them? produced agate and amethyst veins, They do not! The diamonds at the Park have changing chemistry of the also suffered much on their way to the earth’s deposited calcite and barite as veins, and the surface from the mantle and have lost from emplacement of the diatreme introduced 60 to 80 percent of their weight due to many new and exotic minerals, diamond chemical removal processes that occurred being one. Erosion has also been a major during transportation. The dissolved player, removing some 165 feet of material surfaces are slick and have an unusual luster and concentrating the heavy resistant that some describe as oily or greasy, but no minerals in surface soils on the diatreme. oil or grease is on them. Geologists use the terms oily or greasy to describe how light How is conglomerate formed? reflects from the diamond’s surface, and Conglomerate consists of rounded rock someone got confused, perhaps thinking that pebbles and cobbles, typically cemented by oil was actually on them. either oxides or silica. The beds of gravel from which the conglomerate formed What makes a diamond have odd colors? were deposited in one of two environments. For uncut diamonds, colors are dependent Along the 100 million year old coastline, on trace elements that were captured by the heavy seas winnowed out clay and fine sand diamonds as they formed. Color is from beach deposits, leaving a gravel influenced by traces of nitrogen in diamond coastline. As the seas receded or advanced, and whether nitrogen is uniformly gravel beds were formed. Cementation by distributed or present as clumps. Blue dissolved chemicals in groundwater resulted diamonds owe their color to traces of boron. in beds of conglomerate. These beds White light is composed of all the colors we originated after the formation and burial of see in a rainbow and many wavelengths the Prairie Creek diatreme by Late outside our eye’s visible range. What your Cretaceous sediments as they are higher up eye sees as the color of a diamond is due to in the stratigraphic section. Also, the fact that the color is reflected while all Quaternary gravel beds were deposited at other colors of the spectrum are absorbed as the base of major river terraces as the rivers white light passes through the crystal. meandered across their flood plains, at a higher level that the Little Missouri River is For a faceted gem diamond, the colors you now. These basal gravel layers were locally see are due to the ability of the diamond’s cemented by groundwater before the terrace

10 was eroded away, thus leaving behind Can you find diamonds in the surrounding conglomerate masses. The source of the areas? gravel for the Quaternary terrace gravels There are several known smaller pipes on was both the Ouachita Mountain region and private property to the northeast of the Park. the eroding Cretaceous gravel units just These sites have received a considerable mentioned. Both Cretaceous and amount of commercial examination over the Quaternary conglomerates in the area past 25 years. A few diamonds have been consist predominantly of novaculite, chert, recovered, but access to these properties is and sandstone along with some minor denied by the private company that owns amount of quartz vein pebbles and cobbles. them and those individuals and companies holding mineral leases on them. No Search Efforts diamonds have been confirmed as being recovered from other areas adjacent to the Can you break up the lamproite breccia Park, such as Quaternary deposits in and and find diamonds? What about the adjacent to the Little Missouri River, magmatic lamproite? although rumors to such happenings are Early after the discovery of the Prairie Creek often spread. diatreme, one individual was reported to have done just that, but what he broke up Economic Concerns was not hard rock. Instead it was highly weathered material that was essentially soil, Why is the site not commercially mined? but still retained the original breccia rock The Prairie Creek pipe was the first site of texture. Nowadays, it is highly unlikely that commercial diamond in North a visitor would have such luck since the America, but due to financial losses and ground has been repeatedly plowed and under capitalization of the operations, those turned over. As for the magmatic lamproite, efforts were abandoned. Company diamonds are very rarely reported from soils exploration in the 1990’s indicated that the developed from this rock type, so the odds deposit was sub-economic for today’s would be very slim of finding a diamond in finances, due to too few diamonds to make a the hard matrix rock. commercial venture profitable. However, that does not mean that there is less a chance Can you find diamonds in the of finding a diamond than there ever was for conglomerate? visitors. No diamonds to date have been recovered from the conglomerate. The components of When will the diamonds run out? the conglomerate originated from the Statistical information kept by Park Ouachita Mountains to the north during the personnel since 1972 and studies of this and time that the pipe was buried by Cretaceous nearby pipes and the Park’s indicated age sediments. Local river terrace gravels diamond content implies that at the present during the Quaternary also left behind basal rate of recovery by visitors, the Park will gravel deposits. Conglomerate now present continue to have diamonds discovered for at on the surface of the pipe consists of least the next 500 years. And that is for residual masses left behind from gravel diamonds in just the weathered zone, not deposits that have been eroded away. counting those present in the unaltered lamproite tuffs.

11 Mineral Identification the hardness scale. Calcite has a less brilliant luster, has rhombohedral , is 3 on What is calcite? the hardness scale, and fizzes when in Calcite is a mineral composed of , contact with weak hydrochloric acid. carbon, and oxygen. It has a rhombohedral cleavage, is slightly harder than your What is spinel? fingernail, and fizzes when in contact with Spinel is one of the Spinel Group of weak hydrochloric acid. minerals that are composed of metals bound with oxygen; in the case of spinel the metals What exactly is jasper and why is it so are and aluminum. Spinel is a smooth? common high-temperature mineral in Jasper is a fine-grained (cryptocrystalline) metamorphic rocks and in alumina-rich variety of the mineral quartz. It may vary in xenoliths. Another member in the Spinel color from red to tan and has a conchoidal Group of minerals is the iron aluminum . It is smooth because it is hard and oxide named hercynite. It occurs in some has been rounded by transportation in water. basic and ultrabasic igneous rocks. Spinel and hercynite cannot be distinguished easily What is amethyst and why is it purple? as small grains since both are black, have Amethyst is a violet tinted gemstone conchoidal fracture, and have a fairly high consisting of crystalline quartz with iron as density. Both are probably present on the an added impurity. The iron impurity in collecting area. amethyst creates another unstable species in the crystal and this new species jumps to an What is agate and how is it formed? excited state by absorbing a "visible" Agate is a banded type of cryptocrystalline photon. This species is known as a color chalcedony, a variety of quartz. It forms in center. By absorbing a "visible" photon, it layers as a silica gel from silica-laden water makes the crystal appear colored. The naked and converts to agate as the deposits age. eye sees the color as violet. Agate typically displays light and dark banding due to impurities deposited with it. What is mica and why is it shiny? Mica is a group of minerals that have an Misconceptions internal sheet type structure. This structure gives mica a perfect single cleavage How often do you salt the mine? direction. It is shiny because the luster on Despite rumors to the contrary, the site has the broken surface of the sheets or plates is not been nor is salted by Park personnel. brilliant. Phlogopite is the common mica on This is a natural deposit and the diamond the pipe. crystals from this locality have common characteristics that are recognizable to the What is the difference between quartz and experienced individual. It is normal that calcite? very skeptical individuals will always spread Quartz is composed of the elements silicon negative information about the diamond- and oxygen, and is a separate mineral bearing deposit of the Park, perhaps trying species unrelated to calcite, which is to keep other visitors away to increase their composed of the elements calcium, carbon, own chances of recovering a diamond. and oxygen. Quartz typically has a higher luster, has conchoidal fracture, and is 7 on

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References and Reading List

Bush, W. V., Clardy, B. F., Stone, C. G., and Haley, B. R., 1971, Geologic Map of the Murfreesboro Quadrangle, Pike and Hempstead Counties, Arkansas: Arkansas Geological Commission Open-File Report, scale 1:24,000. Dane, C. H., 1929, Upper Cretaceous Formation of Southwestern Arkansas: Arkansas Geological Survey Bull. 1, 215p. Dunn, D. P., 2002, Mineralogy and Geology of the Prairie Creek Lamproite Province, Arkansas – Doctoral Dissertation, University of Texas at Austin, 147 p. Geostor, 2007, DRG24K (Digital Raster Graphic, 1:24,000, USGS) and DOQ (Digital Orthophoto Quadrangle, natural color, 1 meter pixel resolution) acquired from www.geostor.arkansas.gov. Hanson, W. D., Clardy, B. F., Stone, C.G., and Haley, B.R., 1998, Geologic Map of the Murfreesboro Quadrangle, Pike and Hempstead Counties, Arkansas: DGM-AR-00611. Howard, J. M., 1999, Summary of the 1990s Exploration and Testing of the Prairie Creek Diamond-bearing Lamproite Complex, Pike County, Arkansas, with a Field Guide in Contribution to the Geology of Arkansas, Volume IV, AGC MP 18-D, 57-74p. Howard, J. M., 2006, Arkansas Mineral Commodity Database, In-house data: Arkansas Geological Commission. Khokhryakov, A.F., and Pal’yanov, Y.N., 2007, The evolution of diamond morphology in the process of dissolution: Experimental data: American Mineralogist, V. 92, No. 5-6, p. 909- 917. Klein, C., and Hurlbut, C.S., Jr., 1993, Manual of Mineralogy, Twenty-First Edition: John Wiley & Sons, Inc., New York, 681 p. Miser, H. D., and Purdue, A. H., 1919, Gravel Deposits of the DeQueen and Caddo Gap Quadrangles, Arkansas: U.S. Geological Survey, Bulletin 690, 15-29p. Miser, H. D., and Purdue, A. H., 1929, Geology of the DeQueen and Caddo Gap Quadrangles, Arkansas: U.S. Geological Survey, Bulletin 808, 195p., scale 1:125,000. Mitchell, R. H., and Bergman, S. C., 1991, Petrology of Lamproites: Plenum Press, New York and London, 447 p. Morgan, John, 1993, Project Manager’s Report for Phase I – Evaluation program, Crater of Diamonds State Park, Murfreesboro: unpublished file report, Arkansas Department of Parks and Tourism, 77 p. Morris, E. M., 1987, The Cretaceous Arkansas alkalic province; A summary of petrology and in Morris, E. M. and Pasteris, J. D., eds., Mantle and Alkaline Magmatism, The Geological Society of America Special Paper 215, p.217-233. Pantaleo, N. S., Newton, M. G., Gogineni, S. V., Melton, C. E., and Giardini, A. A., 1979, Mineral inclusions in Arkansas diamonds: Their nature and significance: American Mineralogist, V. 64, p. 1059-1062. Scott-Skinner, B. H., and Smith, E. M. W., 1984, A new look at Prairie Creek, Arkansas, in Kornprobst, J., ed. Kimberlites I; Kimberlites and related rocks, Amsterdam, Elsevier, p. 255-283. Wilson, Lionel, and Head III, J.W., 2007, An integrated model of kimberlite ascent and eruption: Nature, V. 447, 5 p. Worthington, Glenn, 2007, Genuine Diamonds found in Arkansas: Mid-America Prospecting, USA, 178 p. Zartman, R. E., 1977, Geochronology of some alkalic rock provinces in eastern and central United States: Annual Review, Earth and Planetary Science, V. 5, 257-286p.

13 Alphabetical Listing of Minerals reported from the Prairie Creek Diatreme and their Geologic Association *

Agate – A banded variety of cryptocrystalline quartz; formed by late hydrothermal activity as vein fillings. Almandine – Garnet variety released into the soil by weathering of crustal xenoliths. Amethyst – A variety of crystalline quartz; formed by late hydrothermal activity as veins and pocket fillings within lamproite breccia tuff. – Garnet variety reported by early worker; possible misidentification. Augite – A pyroxene that is a common constituent of magmatic lamproite. Barite – A barium sulfate present as late hydrothermal veins within tuffaceous rocks. Calcite – Calcium carbonate present as late hydrothermal veins within tuffaceous rocks. Chromite – An oxide that is a constituent of magmatic lamproite; resistant to weathering and often present in soils. Diamond – A constituent of lamproite breccia tuffs and lamproite lapilli tuffs; resistant to weathering and recovered from soils derived by the weathering of these rocks; probably a component of mantle xenoliths. Diopside/Chrome Diopside – A mineral constituent of both magmatic and pyroclastic rock types, consisting of a calcium magnesium silicate. Present as macrocrysts and microphenocrysts. Enstatite – A constituent of magmatic lamproite. Epidote – A constituent of magmatic lamproite; formed during the cooling process. – A rare reaction product within rims of xenoliths. Hydroxylapatite – A phosphate that composes late hydrothermal veins cutting both magmatic and pyroclastic rock types. Generally fine-grained and looks like clay seams. Ilmenite – Present in both magmatic and pyroclastic rock types, the mineral consists of iron titanium oxide. Jasper – A variety of cryptocrystalline quartz; derived from Quaternary river deposits. Jeppeite – A rare reaction product within rims of xenoliths. Kutnohorite (?) – A carbonate reported as an inclusion within lamproite (probable magmatic type). Magnetite – An iron oxide that is a minor constituent of magmatic and pyroclastic rocks. Novaculite – A rock type; a variety of massive microcrystalline quartz, derived from Cretaceous and Quaternary gravels. Olivine – A of magmatic and pyroclastic rock types; magnesium iron silicate. Palygorskite – A secondary clay derived from the weathering of magmatic and pyroclastic rock types. Periclase – A constituent of lamproite breccia tuffs; . – A gem variety of Olivine; scarce; see Olivine. Perovskite – A minor constituent of magmatic and pyroclastic rocks; calcium titanium oxide. Phlogopite – A mica that is a constituent of lamproite lapilli tuffs and, to a lesser degree, lamproite breccia tuffs; often present in soils derived from these rocks; potassium magnesium aluminum silicate. Priderite – A scarce reaction product in the rims of xenoliths. Pyrope – A garnet type that is a minor constituent of magmatic and pyroclastic rock types. Rock Crystal – A colorless variety of crystalline quartz; secondary veins and pocket fillings in lamproite breccia tuffs; present in soils derived from these rocks. . Richerite – An exceptionally rare that is a constituent of magmatic rocks and contains high fluorine content. Serpentine – A group of silicate minerals formed by the weathering of magmatic and pyroclastic rocks; common soil component in shades of green to blue. Spinel – A magnesium aluminum oxide that is a minor constituent of magmatic and pyroclastic rocks; most abundant in soils derived from these rocks. Topaz – A scarce constituent of crustal xenoliths; aluminum silicate. Tremolite – A calcium magnesium silicate formed by the weathering and alteration of magmatic and pyroclastic rock types.

* compiled from Mineral Species of Arkansas, a Digest, 1987, revision of 2007, AGC Bull. 23.

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